That sounds very much like dry rot (serpula lacrymans) spores. If you were to lift the floor boards, your very likley to find a fruiting body and what looks like cotton wool growth. Dry rot has the potential to be quite distructive. Dry rot is usually caused by timbers being on damp, poorly ventilated areas. I would advise you to expose the suspect area and seek specialist advise. its important that affected areas are treated as the fungas has the ability to lay dorment and become re-established. diy preservants are unlikley to work as the chemical concentrate is not usually strong enough
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